Windows: Sync Butler does one thing, and does it pretty well. If you work on different computers, and use a USB ("thumb") drive to move files between them, Sync Butler automates the transfer of chosen folders and recent files between systems.

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You can set how much space on your portable drive is used up by the automatic Sync Butler function. It's (somewhat) illustrated in this demonstration video, which, while light on technical details, is short and funny enough to share here:

So if you've got files big enough to make Dropbox and its ilk expensive or inconvenient, and you like the idea of semi-mindless document syncing, Sync Butler might be worth adding to your bag of tricks. It's a free download for Windows systems, and doesn't require actual installation. Thanks wantafanta for the tip!